AC magnetic inductive field generators are used for several signal applications, including article surveillance. In connection with article surveillance, the AC magnetic field derived from the generator is modified by an object resembling a tuned circuit carried on an article moved through a predetermined region of a retail establishment A receiver coil responds to the modified magnetic field to provide an indication, by activating an alarm, that such an article has been carried through the region.
It is desired for AC inductive magnetic field generators for such surveillance systems, and other systems, to be as inexpensive and efficient as possible. In the past, such magnetic field generators have included relatively expensive power supplies to enable the required AC inductive magnetic field to be derived. Typically, linear power amplifiers have been employed to obtain the desired magnetic field intensity at the required frequencies, which are typically in the 60 KHz range. However, linear amplifiers require large power transformers which increase the size, weight and cost of the AC inductive magnetic field generator.
The size and weight of generators for the required magnetic field can be reduced by utilizing switch-mode amplifiers. A basic difference between a switch-mode amplifier and a linear amplifier is that a linear amplifier continuously stores a large amount of energy, which is released as a function of an input signal. A switch-mode amplifier stores a much smaller amount of energy and releases it at a relatively high frequency. However, switch-mode amplifiers are relatively complex because they require a logic level reference frequency which activates switches of the amplifier, as well as a modulated frequency source.
It is, therefore, an object of the present invention to provide new and improved AC inductive magnetic field generator including a switch-mode device.
Another object of the invention is to provide a new and improved inductive magnetic field generator that is relatively low cost, light weight and which has a small volume so that it can easily be installed in retail establishments as part of article surveillance systems.
A further object of the invention is to provide a new and improved AC inductive magnetic field generator that is powered by a transformerless AC to DC converter and is responsive to only a single frequency determining input.
An additional object of the invention is to provide a new and improved AC inductive magnetic field generator which efficiently converts DC energy from a transformerless AC to DC converter into magnetic field energy in a package having small size, weight and cost.